Java-Stream, toMap with duplicate keys

Question

So there might be one abc for several payments, now I have:

//find abc id for each payment id
Map<Long, Integer> abcIdToPmtId = paymentController.findPaymentsByIds(pmtIds)
          .stream()
          .collect(Collectors.toMap(Payment::getAbcId, Payment::getPaymentId));

But then I reallize this could have duplicate keys, so I want it to return a

Map<Long, List<Integer>> abcIdToPmtIds 

which an entry will contain one abc and his several payments.

I know I might can use groupingBy but then I think I can only get Map<Long, List<Payments>> .

Answer 1

Use the other groupingBy overload.

paymentController.findPaymentsByIds(pmtIds)
      .stream()
      .collect(
          groupingBy(Payment::getAbcId, mapping(Payment::getPaymentId, toList());